Featured page image doesn't display

Hi, it’s Rose. My site: https://capturedesign.pro/

I wonder where the image for each of my feature page comes from? Currently, I add one example image in each page then 2 of them appear, 1 doesn’t. Please help me with this.
And, what if I add more than 1 image to each page, the system will choose the first one to be featured or how?

Thanks!